It does shed light on it, although I forget the details just now. It didn't happen on the set, however it does highlight that there was a specific change in the inflection of how Vader used the word, to taunt Luke.
In fact, there is a running gag of people who accidentally find out that Leia is Luke's sister, even though it was supposed to be kept under tight wraps. One is Anthony Daniels having fallen asleep behind a log, and when he wakes up he hears them talk about it, thinking "Crap, I shouldn't have heard that. I'm just going to pretend to be asleep", as well as someone walking into a post-production screening of the film seeing the revelation. Lucas sat him down and promised her that if she in any way, shape or form revealed this to anyone, he would kill her. :D